This application claims priority to Japanese Patent Application No. P2001-084307.
1. Field of the Invention
This invention relates to a semiconductor photodetector, a semiconductor photo receiver, and a semiconductor device and more particularly relates to a surface illuminated type photodetector in which incident light is cast perpendicularly onto the semiconductor substrate surface and is converted into electrical signals, and a semiconductor photo receiver and a semiconductor device which use the same for optical communications.
2. Description of the Background
In recent years, the need for transmitting large capacity data such as image data has been increasing with the rapid expansion of information services based on communications media such as the Internet. Likewise, there is a need to increase the transmission capacity for such information networks that carry this data.
To construct an optical communications system with a transmission capacity over 10 Gbps, it is necessary to develop an optical transmission device which features ultrahigh speed and high sensitivity. To develop such an optical transmission device, it may be necessary to use an ultrahigh speed, high-sensitivity semiconductor photodetector capable of receiving optical signals and of converting them into electrical signals.
The response velocity of a semiconductor photodetector is determined by the CR constant (calculated as the product of capacitance C and resistance R) and the transit time of the carrier excited by incoming optical signals.
To increase the response velocity, the capacitance C and resistance R must be decreased and the transit time must be shortened. Since the transit time is proportional to the thickness of the photo absorbing layer of the semiconductor photodetector, the photo absorbing layer is preferably thinned as much as possible. However, as the photo absorbing layer becomes thinner, the amount of light that is transmitted but not absorbed by the photo absorbing layer increases, causing a deterioration in sensitivity.
As mentioned above in connection with the thickness of the photo absorbing layer, there is a trade-off between the response velocity and sensitivity, and vice versa. Therefore, it may be difficult to develop a semiconductor photodetector that provides both the desired high response velocity and sensitivity. This has caused a bottleneck in the development of an ultrahigh speed, high sensitivity optical transmission device.
As a conventional solution to the above problem, the method disclosed in JP-A-218488/1993 is known. In this method, a reflector that has a size suitable for the effective detecting area size and consists of two films lying in contact with the semiconductor layerxe2x80x94a dielectric film (lower) and an electrode metal film (upper)xe2x80x94is formed on the side of a substrate that is opposite to its light incidence side and that is reached by incident light passing through the photo absorbing layer. The light that is transmitted but not absorbed by the photo absorbing layer is efficiently reflected back to the semiconductor layer.
FIG. 2 is a schematic sectional view showing a backside illuminated type avalanche multiplication photodetector (APD) based on the above-mentioned art. As shown in the figure, the following layers are consecutively formed on an n-type InP substrate 21 in the following order: a high density n-type InAlAs buffer layer with 0.7 xcexcm thickness 22; a low density n-type InAlAs multiplication layer with 0.2 xcexcm thickness 23; an undoped InGaAs/InAlAs super lattice layer with 0.05 xcexcm thickness 24; a low density p-type InGaAs photo absorbing layer with 1.0 xcexcm thickness 25; a p-type InAlAs buffer layer with 1.0 xcexcm thickness 26; and a high density p-type InGaAs contact layer with 0.1 xcexcm thickness 27. The above composition results in a mesa structure with a diameter of the p-n junction of 50 xcexcm.
The surface of the substrate is thereafter passivated by a SiN insulating film 28, and an n-type ohmic electrode 29 is formed in a desired area on the substrate 21. A p-type ohmic electrode 30 lies not only on the contact layer 27 but also on the SiN insulating film 28 with a 40 xcexcm diameter formed inside the effective detecting area of the contact layer 27.
The dielectric film 28 which consists of a SiN or other similar insulating film hardly reacts with the p-type InGaAs contact layer 26 as a semiconductor layer and p-type ohmic electrode 30 even when it has been annealed at a high temperature in the manufacturing process. Therefore, the uniformity of the interface is maintained at a satisfactory level. The metal surface of the p-type ohmic electrode 30 in contact with the SiN insulating film 28, or a reflector 31 composed of the SiN insulating film 28 and p-type ohmic electrode 30, completely reflects the transmitted light back into the photo absorbing layer 25 with a reflectivity of 100%, which leads to an improvement in quantum efficiency.
As the area of the reflector 31 increases, the effective detecting area size may be larger. Hence, the photodetector""s quantum efficiency depends on the area of the reflector. Also, since an ohmic contact area between the electrode metal film and the semiconductor layer may be formed in the surrounding area (other than the area of the reflector 31), a low reflectivity zone generated by the ohmic contact area does not directly affect the reflection of light to a substantial degree.
The influence of capacitance C may be a more important factor as the transmission capacities in optical communications and other applications increases. Therefore, for the development of a higher speed, sensitive optical transmission device, it is preferable to reduce the photodetector size in order to decrease the capacitance. With respect to the size of the photodetector, the ideal ratio of the minimum effective detecting area size to the diameter of the p-n junction is equal to 1.
If the ratio of the effective detecting area size to the p-n junction diameter is made as near as possible to 1 using the above-mentioned prior art, the size of the ohmic contact area formed in an area other than the reflector area must be decreased in order to obtain a sufficient effective detecting area (area of the reflector), which would result in an increase in the resistance. Conversely, if the ohmic contact area is made sufficient, the effective detecting area size (area of the reflector) should be smaller, resulting in a deterioration in sensitivity.
For example, in the case of the above-described semiconductor photodetector, the diameter of the p-n junction is 50 xcexcm, which makes the capacitance approximately 0.1 pF, and the resulting frequency response at least 10 GHz. Assuming, for example, that this photodetector runs at 40 GHz, the limit for the capacitance is 0.05 pF because of the CR constant, which leads to a calculated result of approximately 34 xcexcm as the optimum diameter of the p-n junction.
Consequently, if a photodetector with the same level of resistance is manufactured using this conventional technique, the effective detecting area size would be 20 xcexcm or less. This not only would necessitate a high accuracy in the optical axis alignment with the fiber in the packaging process but also may cause an optical axis alignment error to occur due to a change in the ambient temperature during use. As a result, the quantum efficiency would likely decrease. Therefore, for a surface illuminated type photodetector manufactured using the conventional technique as explained above, it has been difficult to reduce its size to decrease its capacitance C, which has hampered the development of an ultrahigh speed, high sensitivity optical transmission device.
In at least one embodiment, the present invention preferably provides a high sensitivity, high speed surface illuminated type photodetector that does not cause an increase in the resistance and a decrease in the quantum efficiency although it is smaller than conventional type photodetectors. The present invention may also provide a semiconductor photo receiver and a semiconductor device that incorporate said photodetector.
To address one or more of the above-mentioned limitations in conventional photodetectors, a semiconductor photodetector according to the present invention comprises a surface illuminated type photodetector (a xe2x80x9cbackside illuminated photodetectorxe2x80x9d) that has one or more of the following elements: a plurality of narrow-stripe or dot pattern ohmic contact areas formed in the effective detecting area on the side of the substrate that is opposite to the light incidence side and that is reached by incident light passing through the semiconductor; and a reflector consisting of a transparent film (lower film) and a metal film (upper film) lying on said semiconductor.
It may be preferable from the viewpoint of manufacturing convenience to make the above-mentioned narrow-stripe or dot patterns in the form of concentric rings, but the invention is not limited specifically thereto. Other patterns may be used within the scope of the present invention such as a distribution of independent grid, rectangular or circular patterns.
The size of the above-said narrow-stripe or dot pattern ohmic contact area is preferably determined so that incident light can not resolve the ohmic contact area on the surface. More specifically, because the optical wavelength is approximately 1.5 xcexcm, it is desirable to make the width of each stripe or the diameter of each dot not more than approximately 2 xcexcm so that the incident light can not resolve or detect the xe2x80x9cholesxe2x80x9d in the surface. In this case, the term xe2x80x9capproximatelyxe2x80x9d includes a plus/minus 5% error due to manufacturing limitations around the intended ohmic contact area size of no more than 2 xcexcm.
According to an aspect of the present invention, the surface illuminated type photodetector preferably takes advantage of the nature of light in that it cannot recognize an object smaller than its wavelength and tends to diffuse in the direction of its advance. The above-mentioned narrow-stripe or dot pattern ohmic contact areas function as an electrode and do not function as a reflecting surface. Hence, the light reflecting surface of the surface illuminated type photodetector has an area which is virtually equal to the total of the ohmic contact areas plus the area of the laminate composed of transparent and metal films, that is an effective detecting area.
For example, if circular ohmic contact areas with a diameter of approximately 1.0 xcexcm that are in contact with the semiconductor are distributed inside the reflector surface, the contours of the ohmic contact areas, which are low in reflectivity, are blurred. Due to the diffusion of surrounding light reflected by the reflector, the reflected light inside the effective detecting area, in which both the reflector and ohmic contact areas exist, becomes reflected light which is almost completely governed by bright areas. With virtually no influence from dark areas generated by the ohmic contact areas, the overall reflectivity and the uniformity of sensitivity in the detecting area may be maintained at an adequate level.
The above arrangement preferably ensures that while the effective reflecting area is maintained, the ohmic contact area size is also sufficient. In addition, since the entire effective detecting area is used to make a current passage between the ohmic electrode and semiconductor, a slight decrease in the total ohmic contact area size does not cause a significant increase in the resistance that may affect the photodetector operation. For this reason, the speed can be increased by decreasing the resistance between electrodes to reduce the RC constant.
At the same time, since the effective reflecting area is not affected by the ohmic contact areas, the quantum efficiency or sensitivity may be enhanced by increasing the amount of reflected light. In other words, it may be possible to make a smaller semiconductor photodetector without an increase in the CR constant. The corresponding reduction in capacitance C permits the development of a semiconductor photodetector with a high response velocity and a high sensitivity as well as a semiconductor photo receiver and a semiconductor device which incorporate the same.